The main objective of the project is to explore the validity of some of the experimental support of the immune surveillance theory. Especially the effects of immunodeficiencies or immunosuppression on tumor development either spontaneous or after exposure to chemical or viral oncogenic agents. Additional aspects to be studied include the direct effect of the oncogenic agent on the immune response (i.e. the carcinogen as immunodepressant), the effects of immunodeficiencies or immunosuppression on the genetic susceptibility or resistance of inbred mouse strains to chemical or viral oncogenic agents and the role of carcinogen-host and tumor-host interactions on tumor development and progression. The main model to be used will be the nude-athymic mouse which does not develop a functional thymic rudiment and has a profound immune deficiency. These studies will help to understand the role of the host immune respones on susceptibility to tumor development.